It is a current trend in the audio industry to use digital signal processing for improving the sound quality of consumer audio reproduction systems. Typical shortcomings of compact or inexpensive audio reproduction systems include for example non-flat magnitude response, modest ability to radiate low frequencies and noise. Another undesirable property of many such systems is the occurrence of non-linear distortion which may cause sound characteristics such as timbre and phase to change as a function of the sound amplitude. Non-linear distortion may also cause a non-linear magnitude response (with respect to intensity) at a given frequency, and may create new frequency components in the sound output, for example, at integer multiples of the frequencies of input frequency components (harmonics). As a result, playing two identical sounds with different playback gains on a non-linearly distorting audio reproduction system may result in two very different output sounds. A typical source of non-linear distortion in a commercial sound reproduction system is forcing a loudspeaker unit to operate outside its linear operation range in order to achieve a louder sound output without increasing costs for the transducer.
One way to suppress non-linear distortion is to attenuate the input signal before it is fed to the loudspeaker, but this also makes the system quieter, which is typically not acceptable. An approach to suppressing non-linear distortion while sacrificing only a little of the system's loudness is to use a dynamic range limiter which monitors the input signal and applies attenuation only for the loud parts, so that the amplitude of the signal fed to the loudspeaker never exceeds a given threshold value.
Since non-linear distortion is typically frequency-dependent, it is often desirable to apply the limiting separately to different frequency bands, so that only problematic frequency and signal magnitude combinations are attenuated. This approach, called multi-band limiting/compression, also alleviates some of the side effects related to dynamic range compression, such as ‘volume pumping’.
However, as multi-band limiters/compressors attenuate different frequencies differently depending on the input signal, this may lead to perceptually disturbing timbral changes. In US Patent Application Publication No. 2013/0013096 A1, incorporated in its entirety herein, a multi-band compressor with timbre preservation is proposed to reduce such timbral changes.
As described above, several methods have been proposed for suppressing non-linear distortion occurring in an audio reproduction system, so as to improve sound quality of the audio reproduction system. However, it would be desirable to provide devices and/or methods mitigating at least some of the above described problems associated with suppression of non-linear distortion.
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